Coal is a widely used, but limited, fuel for generating electricity in the United States and around the world. However when burned, coal can emit significant amounts of pollutants which create environmental problems. Environmental concern is exemplified by the Clean Air Act Amendments of 1990 creating new emissions limitations for coal of 2.5 pounds of sulfur dioxide per million BTU effective in 1995 and 1.2 pounds of sulfur dioxide per million BTU effective in the year 2000.
A utility that burns high sulfur coal currently has the option of switching to a low sulfur coal or scrubbing flue gases to remove sulfur dioxide. Scrubbing sulfur dioxide requires significant capital investment and is operationally expensive. For many utilities, switching to low sulfur coal would be very expensive due to transportation costs for delivering coal from a distant source and capital costs associated with plant modification to accommodate coals with different combustion characteristics. Substantial deposits of high sulfur coal currently fuel many electrical power generation plants. A need exists to improve the cleaning of sulfur from such coals prior to combustion so that they may be efficiently used without producing excessive pollutants.
Beneficiation of coal refers to the removal of non-coal material from raw coal to produce a relatively clean coal product. Raw coal is composed of high purity coal material and non-coal material. Non-coal material in coal, commonly referred to as ash, normally includes pyrite, clays, and other aluminosilicate materials. The presence of large amounts of these ash materials can create problems during combustion, such as slagging and fouling. Sulfur is present in raw coal in two forms, organic sulfur and inorganic sulfur. Organic sulfur is chemically bound as part of the coal matrix. Inorganic sulfur is all sulfur not chemically bound in the coal matrix. Pyrite sulfur is the predominate form of inorganic sulfur. Sulfate sulfur is another form of inorganic sulfur associated with ash forming materials. Physical beneficiation effectively removes only inorganic sulfur. Processes for beneficiating coal are varied, but commonly utilize dense medium separation, jigs, or froth flotation to separate clean coal from non-coal material. Because of its versatility, high efficiency and ease of operation, dense medium separation is perhaps the preferred separation technique.
In dense medium separation, raw coal is introduced into a medium having a specific gravity intermediate between that of coal and non-coal material. The dense medium may be a homogeneous liquid, but is more often composed of water and magnetic particles, such as ferromagnetic particles. Magnetite is a commonly used magnetic particle. Separation can be carried out in a dense media bath or tank, or in a cyclone. When a cyclone is used, coal is generally removed as the overflow product while refuse becomes the underflow product. After separation of coal and refuse, it is advantageous to recover the magnetic particles from the coal and from the refuse for reuse.
Raw coal feed, typically known as run-of-mine coal, is a mixture of three components, namely organic material, rock and pyrite. In raw coal, some particles are liberated, meaning that they constitute relatively pure components. Other particles are locked, meaning that these particles contain two or more of the three components locked together. Such locked particles are referred to as middlings.
Each of the raw coal components has a characteristic specific gravity. To illustrate, organic material has a specific gravity of about 1.25, rock has a specific gravity of about 2.85 and pyrite has a specific gravity of about 5.0. A raw coal feed contains particles with many specific gravities as a result of the differing specific gravities of the three separate components and the combination of components which are locked together.
While dense medium beneficiation has been effective for large size coal feed particles, those greater than approximately 0.5 mm in size, it has typically not been used for smaller-size coal particles. In this regard, the separation efficiency for small particle coal feeds has not been satisfactory. As a result, small coal particles are often discarded.
One way to improve the separation of coal from non-coal material is to crush or otherwise comminute the raw feed to liberate high purity coal and non-coal material in the middlings. Generally, as the average size of the particles in the raw coal feed becomes smaller, more coal and non-coal material are liberated and the percentage of particles constituting middling decreases, potentially allowing the recovery of more coal product. Crushing or grinding a coal feed to liberate coal locked with non-coal material in middlings has not been practical because there was no process for treating fines which efficiently separates coal from non-coal material. Middlings material, therefore, either reports to the clean coal, which introduces pyrite and other unwanted minerals into the fuel, or reports to the refuse resulting in an undesirable loss of coal. Comminution of an entire coal feed is, however, costly and not commercially practical. The expense of comminution is significant and it would be desirable to minimize the costs.
As indicated above, in order to recover coal from middlings to produce a high purity coal product, it is necessary to comminute the middlings and then to separate the coal from refuse. If middlings are not processed for further coal recovery, a substantial quantity of useable coal in the middlings will be discarded along with non-coal material. Accordingly, to maximize recovery of a clean coal product, it is essential to develop beneficiation processes designed to handle small particle raw coal feed.
U.S. Pat. No. 4,364,822, by Rich, issued Dec. 21, 1982, describes a coal cleaning process involving two-stage cyclone separation that produces three products, clean coal, refuse, and middlings. Middlings are then crushed and recycled through the cyclones with the raw coal feed. Rich, however, specifically teaches away from a dense medium process using magnetic particles based on problems with the recovery of magnetic particles.
U.S. Pat. No. 3,908,912 by Irons, issued Sep. 30, 1975, describes a process whereby refuse is initially separated out at high density, followed by a lower density separation to yield clean coal and middlings. Middlings are then crushed for further cleaning. However, in Irons small size coal is not removed from the coal feed prior to the initial high density separation which results in additional refuse in the clean coal product. Moreover, Irons discloses that cyclone separations of small coal fines are inefficient in that particles are frequently misplaced. As such, Irons teaches the use of secondary cyclones followed by flotation to eliminate refuse in the coal.
Many attempts have been made to clean fine particle coal, with varying results. In dense medium cycloning, separation efficiency drops as coal feed particles become smaller. In particular, considerable difficulty is encountered in cleaning a coal feed made up of particles less than about 0.5 mm in size. Also, recovery of the magnetic particles from the dense medium after beneficiation becomes more difficult as coal feed particles become smaller.
Accordingly, there is a need for an effective and efficient means for beneficiating coal feed particles less than about 0.5 mm in size where the separation efficiency is sufficient such that the coal product meets desired specifications. The separation efficiency of a coal cleaning process is frequently illustrated through probability curves known as partition curves. These curves describe the probability that a given particle in the feed will report to the clean coal rather than refuse. The measure of the slope of the vertical portion of a partition curve is the separation's probable error, or Ep. The more vertical the center portion of the partition curve, the more efficient the separation and the smaller the probable error.
In order to avoid the difficulties associated with cleaning small size particles, many methods for processing fine coal particles discard particles below a threshold size prior to beneficiation typically referred to as desliming. Desliming has traditionally been based o limitations of the beneficiation process. For example, U.S. Pat. No. 3,794,162 by Miller et al., issued Feb. 26, 1974, discloses a heavy medium beneficiating process for particles down to 150 mesh (0.105 mm). Particles smaller than 150 mesh are screened-out prior to beneficiation by dense medium cyclone. U.S. Pat. No. 4,282,088 by Ennis, issued Aug. 4, 1981, discloses a process where particles smaller than 0.1 mm are separated out in a cyclone classifier and discarded prior to beneficiation by dense medium cyclone. When all particles below 0.1 mm or 0.105 mm in size are discarded, pure coal is also discarded both as small coal particles and as coal locked in small middling particles.
The ability to deslime by screening or sieving is limited by available screen and sieve construction. Screening or sieving large quantities of material below a size of about 150 mesh is not practical. Classifying cyclones, which separate particles based on different particle settling velocities, have been used to classify coal feeds, but have not been effective for making a size classification of coal feed at 0.015 mm. Rejecting only the smallest coal particles in raw coal feed, on the order of 0.015 mm and smaller, presents a major problem. Particles smaller than this size are predominately refuse material which should be discarded.
One parameter in cyclone design which has received relatively little attention is the size of the inlet orifice through which feed enters the cyclone. Arterburn, in a paper entitled, "The Sizing of Hydrocyclones" (Krebbs Engineers 1976), notes that feed orifices usually have an area between 6 percent and 8 percent of the area of the cyclone feed chamber. The modification of inlet diameters has not been identified as a factor to improve a classifying cyclone separation capability.
Multiple classifying cyclones arranged in a countercurrent flow circuit have been used for size classification of starch. U.S. Pat. No. 4,282,232 by Best, issued Aug. 11, 1981, describes a countercurrent cyclone circuit designed primarily to wash starch. As far as the inventor knows, a countercurrent arrangement of classifying cyclones is not practiced in the coal cleaning industry and has not been used to separate particles of the magnitude of 0.015 mm and smaller.
Attempts have been made in the coal industry to eliminate the need for desliming by improving the beneficiation process. For example, U.S. Pat. No. 4,802,976, by Miller, issued Feb. 7, 1989, discloses a process in which froth flotation is used to recover coal particles smaller than 28 mesh (0.595 mm) downstream of a dense media cyclone. But this process is not appropriate for all coals. A raw coal feed often contains oxidized coals which do not float. Also, pyrite tends to float, along with clean coal, thereby contaminating the clean coal product. Devising a process to treat all types of fine particle coal and to effectively remove pyrite from the smallest size fractions, has been problematic.
Cyclones for use in connection with dense medium beneficiation have varying size parameters and can be subject to varying operating conditions. In general, cyclones do not operate as effectively when used to beneficiate small size particles. A problem with the use of cyclones for the beneficiation of small coal particles is the need to assure that the particles correctly report to either the underflow as refuse or overflow as coal. Small particles often become misplaced, thereby decreasing the separation efficiency of the cyclone.
One cyclone parameter is the area of the inlet orifice through which raw coal feed enters the cyclone. U.S. Pat. No. 2,819,795 by Fontein, issued Jan. 14, 1958, discloses a cyclone design where the area of the inlet is calculated to equal between 0.1 and 0.4 times the area available for overflow. Fontein also specifies a cyclone diameter between two and three times the diameter of the overflow. Fontein does not discuss the inlet diameter as related to the cyclone diameter or particle velocity. U.S. Pat. No. 4,341,382 by Liller, issued Jul. 27, 1982, discloses a design for an eighteen inch diameter cyclone where the inlet tube diameter is calculated to equal between 0.25 and 0.35 times the cyclone diameter.
Fourie et al., "The Beneficiation of Fine Coal by Dense-Medium Cyclones", Journal of South African Institute of Mining and Metallurgy, pp. 357-361 (October 1980), discloses the use of magnetite particles in beneficiating minus 0.5 mm coal by dense medium cycloning where at least 50 percent of the magnetite is finer than 10 microns (0.010 mm). Finer size magnetite is, however, more difficult and costly to recover from clean coal and refuse. Fourie discloses the recovery of magnetite in a rougher-cleaner-scavenger arrangement of wet drum magnetic separators and reported serious problems with magnetite loss. There is a need for a process which employs magnetite small enough to separate fine size coal and refuse effectively, but allows for sufficient recovery of magnetite after beneficiation.
Magnetite used in Fourie was prepared by milling magnetite ore. But milling ore to ultra-fine sizes is very expensive, and milling gives little control over particle size distribution. Magnetite for use in dense media separation can also be produced by chemical reduction of hematite. U.S. Pat. No. 4,436,681 by Barczak, issued Mar. 13, 1984, discloses a process whereby hematite prepared by spray roasting of iron chloride is reduced to magnetite. However, Barczak does not discuss magnetite particle size or recognize problems encountered during magnetite recovery following dense medium separation.
U.S. Pat. No. 4,777,031 by Senecal, issued Oct. 11, 1988, discloses a process whereby magnetite is produced by pyrohydrolysis of iron chloride at temperatures between 1000.degree. C. and 1600.degree. C. However, Senecal is directed to producing magnetite particles between 0.02 and 0.2 microns (0.00002 mm to 0.0002 mm) in size that are well suited for binder systems such as those used in magnetic recording media. Senecal's process results in magnetite particles too small to be used effectively in dense medium separation of coal due to problems with recovering such small particles following dense medium separation.
Magnetite used in dense medium separation has traditionally been recovered for reuse by first draining the medium from the separated product over screens and then rinsing the product over screens to remove the remaining magnetite. Magnetite is then separated from the rinse water, dilute medium, by magnetic separation. However, when cleaning fine size coal particles, screens are not effective to keep coal and refuse particles from passing through with the medium and rinse water. These fine particles of coal and non-coal material contaminate the dense medium and are difficult to separate from the magnetite in conventional magnetic drum separators.
Another problem with the recovery of small magnetite particles is that it is difficult to separate the magnetite from rinse water by magnetic separation. U.S. Pat. No. 4,802,976 by Miller, issued Feb. 7, 1989, proposes recovering magnetite as the sink from froth flotation cells, thereby avoiding the problem of fine coal and non-coal particles entrained with magnetite during magnetic separation. Froth flotation systems are, however, complex and difficult to operate. The use of a magnetic separator incorporating a high density gradient magnet in a matrix design could be employed. However, high density gradient magnets are expensive and matrix separators complicate operation compared to traditional magnetic drum separators. There is a need for an effective separation process using easier to operate magnetic separators and more economical designs for magnetic separation.
In order to satisfy utility combustion requirements, the clean coal product from beneficiation must be dewatered to reduce its moisture content. Fine particle coal is more difficult to dewater than larger-size coal because of its greater surface area.
In light of the foregoing, what is needed is an improved process for beneficiating fine particulate coal such that desired specifications, such as sulfur content, can be met. Many of the problems impeding development of such a process have been described, and they are formidable. A need exists for a process that maximizes the recovery of coal without the expense of comminuting the entire coal feed. Also, methods of classifying coal particles based on size must be improved, particularly methods using classifying cyclones. Improved separation efficiency of fine particle coal in high throughput dense medium cyclones is desired. Methods to effectively recover ultra-fine size magnetic particles for reuse following dense medium separation are needed to improve the viability of dense medium separation of fine particle coal. Improved methods are also needed for producing magnetic particles of optimum size to effect good dense medium separation while maximizing magnetic particle recovery.